[SYS] Password management improvements (#1787)

* Use one single password for Access point, OTA and WebUI access
* Enable to change the password during WM onboarding, with the WebUI, and MQTT command
* Remove the default password during the first configuration to connect to the AP
* When using the last 8 digits of the MAC as the password, it can now be changed
* Remove gateway password sending to client

docs/upload/builds.md

@@ -90,7 +90,7 @@ build_flags =
   '-DMQTT_USER="mqttusername"'
   '-DMQTT_PASS="mqttpassword"'
   '-DMQTT_SERVER="mqttserver.local"'
-  '-Dota_password="otapassword"'
+  '-Dgw_password="12345678"'
   '-DLED_RECEIVE=LED_BUILTIN'        ; Comment 1
   '-DLED_RECEIVE_ON=LOW'             ; Comment 2
   '-DRF_RECEIVER_GPIO=13'

docs/upload/portal.md

@@ -4,7 +4,7 @@
 
 Once loaded into your ESP, and if you don't use the manual configuration, you have to set your network parameters with WiFi Manager portal.
 
-From your smartphone, search for  OpenMQTTGateway or OMG_ WiFi network and connect to it with the `WifiManager_password` you have defined (the default password is **"your_password"**), or if you are using a device sold by Theengs or the macro `-DWM_PWD_FROM_MAC=true`, the password will be the last eight digits of the device MAC Address with upper case.
+From your smartphone, search for  OpenMQTTGateway or OMG_ WiFi network and connect to it without password, or if you are using a device sold by Theengs or the macro `-DWM_PWD_FROM_MAC=true`, the password will be the last eight digits of the device MAC Address with upper case.
 Example, the password would be `CCDDEEFF` for a MAC Address `AABBCCDDEEFF`. 
 For the Theengs Plug, the MAC Address can be found on the device sticker, and for all the devices, it is printed on the serial monitor logs.
 
@@ -35,8 +35,10 @@ If the board is connected by ethernet, the Wifi and password can be empty. If yo
 * Set your MQTT Server password (optional, 64 characters maximum)
 * Set your MQTT base topic if you need to change it (you must keep the / at the end) (default: home/, 64 characters maximum)
 * Set your gateway name if you need to change it
-* Set your Over The Air password `ota_password`, this password is used for local and remote OTA - it is also the password being used for the WebUI login. (default: OTAPASSWORD,  64 characters maximum)
-
+* Set your gateway password (default: empty,  8 characters minimum, 64 maximum), this password is used for 
+  * Local and remote OTA
+  * WebUI login (login:"admin", password: "")
+  * WiFi Manager Access Point password after initial config
 * Click on save
 
 ![WiFi manager save](../img/OpenMQTTGateway_Wifi_Manager_save.png)